Multi-pixel x-ray source with tungsten-diamond transmission target

ABSTRACT

A multi-pixel x-ray source is provided. The x-ray source includes a plurality of transmission target assemblies. The transmission target assembly includes a tungsten target and a diamond substrate. The substrate includes a first transmission surface and a second transmission surface opposite first transmission surface. The substrate further includes a first side surface and a second side surface disposed between the first and second transmission surfaces. The target covers the first transmission surface of the substrate. The transmission target assembly further includes a base. The base surrounds the first and second side surfaces of substrate, exposing a collimator surface of the second transmission surface and the target. The transmission target assembly is configured to transmit x-ray generated by the target through the target and the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/667,929, filed May 7, 2018, entitled “MULTI-PIXEL X-RAY SOURCEWITH W-DIAMOND TRANSMISSION TARGET,” which is hereby incorporated in itsentirety herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support under Grant No.1R03EB024952-01 awarded by National Institutes of Health. The governmenthas certain rights in the invention.

BACKGROUND

Kilovoltage X-ray imaging is one of the most common diagnostic imagingmodality in radiology as well as image guided intervention andradiotherapy. In x-ray generation, a high voltage is used to accelerateelectrons released by a cathode to a high velocity and the high-velocityelectrons collide with a target on an anode, creating x-rays. One of thepredominant x-ray production processes is Bremsstrahlung interactionprocess, where radiation is given off by electrons as they are scatteredby the strong electric field near high-Z (proton number) nuclei. Thisprocess is highly inefficient, where only 1% of the energy is convertedto x-ray photons. The rest of the electrons' kinetic energy is convertedto heat, deposited on the target, and eventually dissipated to theenvironment.

BRIEF DESCRIPTION

In one aspect, a multi-pixel x-ray source is provided. The x-ray sourceincludes a plurality of transmission target assemblies. The transmissiontarget assembly includes a tungsten target and a diamond substrate. Thesubstrate includes a first transmission surface and a secondtransmission surface opposite first transmission surface. The substratefurther includes a first side surface and a second side surface disposedbetween the first and second transmission surfaces. The target coversthe first transmission surface of the substrate. The transmission targetassembly further includes a base. The base surrounds the first andsecond side surfaces of substrate, exposing a collimator surface of thesecond transmission surface and the target. The transmission targetassembly is configured to transmit x-ray generated by the target throughthe target and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described below illustrate various aspects of thedisclosure.

FIG. 1 shows the three key heat transfer steps in an x-ray tube.

FIG. 2A shows a schematic diagram of the cross-section of a transmissiontarget.

FIG. 2B shows a schematic diagram of the cross-section of a reflectiontarget.

FIG. 3A shows a simulation model of x-ray generation using atransmission target.

FIG. 3B shows a simulation model of x-ray generation using a reflectiontarget.

FIG. 4A shows simulated trajectories of 120 keV electrons in atransmission target.

FIG. 4B shows simulation results of energy deposition as a function ofdepth for 80 keV, 100 keV and 120 keV electrons.

FIG. 5A shows transmission x-ray fluence from a transmission target as afunction of tungsten thickness with different electron beam energies.

FIG. 5B shows energy fluence of transmission x-rays from a transmissiontarget as a function of tungsten thickness with electron beam energy at120 keV.

FIG. 6 shows optimal thickness of the tungsten (W) layer in atransmission target as a function of electron energy.

FIG. 7 shows fluence efficiency of transmission targets having tungstenthicknesses from 1 μm to 11 μm as a function of electron beam energies.

FIG. 8 shows x-ray spectra of a 5 μm thick transmission target and a 5mm thick reflection target as a function of energy.

FIG. 9 shows x-ray fluence profiles from transmission targets havingdifferent thicknesses and x-ray fluence profile of a reflection target,where the electron beam energy is at 120 keV.

FIG. 10A is a schematic diagram of the transmission target model used inthe simulations.

FIG. 10B shows simulated temperature distribution near the focal spot onthe W-diamond transmission target shown in FIG. 10A, where a 3 ms, 11 kWbeam is used in the simulations.

FIG. 10C shows simulated temperature distribution in the x-y plane ofthe target shown in FIG. 10A with a 3 ms, 11 kW beam.

FIG. 11A shows variation of the maximum focal spot temperatures versustime for a transmission target.

FIG. 11B shows variation of the maximum focal spot temperatures versustime for a reflection target.

FIG. 12 shows maximum electron beam power as a function of pulse widthfor a transmission target and a reflection target.

FIG. 13A shows a schematic diagram of a tungsten-annealed pyrolyticgraphite (W-APG) laminate target on an annealed pyrolytic graphite (APG)anode base.

FIG. 13B shows the thermal conductivity of APG compared to othermaterials.

FIG. 14A shows an exemplary multi-pixel thermionic emission x-ray(MPTEX) prototype.

FIG. 14B shows a functional diagram of the prototype shown in FIG. 14A.

FIG. 15A shows the temperature distribution of a tungsten target,generated by a simulation.

FIG. 15B shows temperature distributions of a dual layer W-PG targetwith the pyrolytic graphite (PG) having a low a-b plane thermalconductivity.

FIG. 15C shows temperature distributions of a dual layer W-PG targetwith the PG having a high thermal conductivity.

FIG. 15D shows maximum power allowed for the three types of anodesillustrated in FIGS. 15A-15C, while keeping focal spot temperature under3000° C.

FIG. 16A shows a cross section of the MPTEX tube shown in FIG. 14A.

FIG. 16B shows the APG anode plate of the tube shown in FIG. 16A.

FIG. 17 shows CVD apertures for W and PG deposition.

FIG. 18 shows a block diagram illustrating a computing device inaccordance with an aspect of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to a high brightness andhigh-efficiency x-ray source. The x-ray source may be used fortetrahedron beam computed tomography (TBCT). The present disclosure isbased, at least in part on, a tungsten diamond or pyrolytic graphitelaminated target. As shown herein, a W-diamond target configuration canbe used to improve the tube power, such as by improving the focal spotpower density. Further, a multiple-pixel x-ray source using atransmission target eases the geometry design of an x-ray tube andincreases the tube power with multiple pixels.

Because of the inefficiency in the x-ray generation processes, amajority of electrons' kinetic energy is converted to heat. Heatmanagement is therefore important in x-ray tube design to protect thex-ray generating target.

FIG. 1 illustrates the three major steps of heat transfer in an x-raytube. Electrons' kinetic energy is converted to heat at the focal spot,the location on the target where the electrons hit, when the highvelocity electrons hit the target. The heat is conducted to the anodebody that surrounds the target. The heat is then radiated if a rotatinganode is used, or conducted if a fixed anode is used, to the housing ofthe x-ray generation tube or tube housing. The heat is then dissipatedto the atmosphere through the tube housing.

With large amount of heat deposited on a small spot size, the maximumfocal spot power density of x-ray tubes is about 0.5-1 kW/mm2, limitedby the thermal conductivity of the target material, before the targetmelts. Modern x-ray tubes may have a peak power as high as 100 kW, thusthermal management is very challenging in the design of x-ray sources.Most x-ray tubes use a rotating anode to spread the heat from the smallfocal spot to a larger area. Even so, high power x-ray tubes still needa relatively large focal spot size to prevent the tungsten target frombeing damaged by the heat. A rotating anode used to manage heat does notwork well for multi-pixel x-ray sources, because of the sources'elongated geometries. With the heat distributed to a plurality of focalspot positions, multi-pixel x-ray sources may use a fixed anode that mayconduct the heat to outside the tube at a fast speed, however, the totalarea of focal spots is still limited.

Further, focal spot power density also affects image resolution. Theimage resolution of x-ray systems is determined jointly by focal spotsize and detector pixel size. While current x-ray detector technologycan fabricate detectors with very small pixel size, the focal spot sizeof x-ray sources remains at ˜1 mm due to the limitation of focal spotpower density, as described above. Image resolution is also importantfor CT, as well as for other x-ray imaging systems.

Besides image resolutions, x-ray tube focal spot power density alsolimits imaging speed, especially for x-ray scanners with fixed anodetubes, such as micro-focus and multi-pixel x-ray sources. Focal spotpower density also limits CT imaging speed, especially forinverse-geometry CT15 and TBCT, where the multi-pixel x-ray sources usea fixed anode. Due to limited multi-pixel thermionic emission x-ray(MPTEX) tube output, slow rotating gantry machines, such as c-armLinacs, are used in TBCT.

Limited focal spot power density imposes an even more significantconstraint on phase-contrast CT and micro-focus x-ray sources that areused in small animal imaging platforms. In phase-contrast CT, a gratinginterferometer is often used to produce coherent x-ray beams. X-rayphotons are largely blocked by the grating interferometer, thus a muchhigher flux is needed from x-ray sources in phase-contrast imaging. Inmicro-focus x-ray sources, the electron beam is focused to a tiny spotsmaller than 100 μm for small animal imaging. Micro-focus x-ray sourcesalso have to use a fixed anode due to the long scanning time.

Accordingly, limited focal spot power density of x-ray sources becomes amajor roadblock in the further advancement of x-ray imagingtechnologies. The high output of MPTEX source with the anodes disclosedherein allows faster imaging speed and higher image resolution.

Due to engineering difficulties, multi-pixel x-ray sources have multipletargets that are spatially distributed. A multi-pixel x-ray source canalso be referred to as a distributed x-ray source. A multi-pixel x-raysource uses a stationary reflecting anode. Unlike conventional x-raytubes with a single focal spot, the heat of distributed x-ray source isdistributed to a plurality of focal spot positions. The total area offocal spots is, however, still limited. A multi-pixel x-ray sourceoperates in a pulse mode. The x-ray pixels are activated sequentiallywith a short dwell duration. Oosterkamp described the power limitationof the x-ray target as,

$\begin{matrix}{{P = {\frac{\Delta {TA}}{2}\sqrt{\frac{{\pi\lambda\rho}c}{t}}}},} & (1)\end{matrix}$

where P is the electron beam power deposited in the focal spot, ΔT isthe temperature rise, A is the focal spot area, t is the dwell duration,and λ, ρ and c are respectively the thermal conductivity, density, andspecific heat of the target material. The maximum power allowed isinversely proportional to the square root of dwell duration.

By rotating the anode, the dwell duration of electrons reduces withincrease in rotation speed of the anode. Development of multi-pixelx-ray sources with a rotating anode is challenging due to its elongatedanode geometry. Multi-pixel x-ray sources can reduce dwell duration byincreasing the scanning speed. The scanning speed, however, is limitedby the imaging detector readout speed. For rotating anode x-ray sources,the anode rotates around the x-ray focal spot while the x-ray focal spotremains at the same position, and thus the detector integration time isindependent of the anode rotation speed. For multi-pixel x-ray sources,the data of each x-ray pulse need to be differentiated as they representsampling at different locations. The x-ray detectors used by CT scannerscan be read out at about 10,000-20,000 samples per second, which limitsthe minimum dwell duration to about 50 μs for multi-pixel x-ray sources.

Enhanced thermal performance of the x-ray target material describedherein is used to increase the focal spot power density. According tothe Equation 1, the power of an x-ray source is proportional to thesquare root of the thermal conductivity of the target material, such astungsten (W). Diamond has the highest thermal conductivity (2200 W/mK)among all known materials. But, because of the low-Z number of carbonatoms, diamond is inefficient in x-ray production. The two materials arecombined to improve x-ray production. In one embodiment, a thin layer oftungsten is deposited or grown on the diamond substrate to improve x-rayproduction. Alternatively, diamond is grown on tungsten substrate. Insome embodiments, chemical vapor deposition (CVD) techniques are used toachieve diamond thickness on the order of few mm with a high growthrate. The temperature of the substrate for growing diamond is kept above700° C. to enhance the growth of diamond crystals and also suppress thegrowth of graphite. Tungsten is used as substrate materials where alocalized carbide layer of a few nm is formed. Diamond crystals can begrown on diamond and non-diamond substrate like copper, gold, silicon ortungsten by chemical transport in a closed system. Substrates made ofcarbides such as SiC, WC and TiC are particularly suitable for diamonddeposition. A pressure vapor deposition technique may be used tofabricate a W-diamond target.

FIG. 2A illustrates a cross-sectional view of a transmission targetassembly 200. Transmission target assembly 200 may correspond to asingle x-ray source pixel in a multi-pixel x-ray source array. Forcomparison, a reflection target assembly 202 is shown in FIG. 2B. Intransmission target assembly 200, x-rays pass through the target. Inreflection target assembly 202, x-rays are reflected by the target.

In the exemplary embodiment, a transmission target assembly 200comprises a target or transmission target 204 and a substrate 206.Target 204 may be made of tungsten. Substrate 206 may be made ofdiamond. Substrate 206 comprises a first transmission surface 208 and asecond transmission surface 210 opposite first transmission surface 208.Substrate 206 further comprises a first side surface 214 and a secondside surface 216. First and second side surfaces 214, 216 are disposedbetween first and second transmission surfaces 208, 210. Target 204covers first transmission surface 208 of substrate 206. Transmissiontarget assembly 200 further comprises a base 212. Base 212 may be madeof copper. Base 212 surrounds first and second side surfaces 214, 216 ofsubstrate 206, exposing a collimator surface 218 of second transmissionsurface 210 and target 204. Base 212 may form a collimator edge 220.Target 204 may be made of a thin tungsten layer deposited on a diamondsubstrate 206 brazed on a copper base 212. Generated x-rays aretransmitted through target 204 and substrate 206, and further throughcollimator surface 218. Collimator edge 220 of base 212 forms acollimator and guide the generated x-ray into a fan or cone shape.

In comparison, reflection target assembly 202 comprises a reflectiontarget or target 222 and a base 224. Base 224 surrounds all outersurfaces of target 222, except a reflection surface 226. Target 222 maybe made of a thick tungsten slab embedded in copper base 224. Thegenerated x-rays are reflected from target 226 and received by adetector.

A transmission x-ray target produces uniform x-ray beam intensitywithout producing “heel-effect” characteristic of a reflective target,where only fluence at a large angle can be used. It also allows morecompact tube geometry, which is important for multi-pixel x-ray sources.The tungsten layer is thin enough such that the diamond target is usedfor transmission.

In one embodiment, transmission target assembly 200 comprises a thintungsten film deposited on a ˜2 mm thick diamond substrate brazed on acopper or graphite base. Diamond is stable at high temperature in vacuumenvironment. Its high thermal conductivity allows fast heat removal fromtarget 204 and its low atomic number results in low x-ray attenuationand low Bremsstrahlung yield.

The copper base 212 of the transmission target 204 not only allows forfast heat removal from the target 204, but also collimates the beamsinto cone- or fan-shaped beams. Primary collimation close to the targetallows a multi-pixel x-ray source with finer pixel spacing.

In the exemplary embodiment, the diamond transmission target isevaluated and optimized using Monte Carlo and finite element thermalsimulations.

Monte Carlo (MC) Simulations of Energy Deposition

In the exemplary embodiment, transmission target assembly 200 comprisesa thin layer of tungsten (W) deposited on a diamond substrate. Thethickness of W in the transmission target was optimized using Geant4Monte Carlo (MC) simulations. A transient thermal model was built in afinite element analysis software. Finite element thermal simulationswere performed to evaluate temperature distributions in the target underdifferent power loadings. The maximum allowed power while keeping thetarget temperature below 3000° C. was determined for different pulsewidths. The x-ray fluence and thermal performance of the transmissiontarget were compared to that of a reflection target.

Electrons impinging on the target deposit their energy at various depthsin the target. To model the energy deposition for the purpose of thermalanalysis, MC simulations were performed using Geant4 simulation toolkit.Energy deposition of 80 keV, 100 keV and 120 keV electrons were obtainedas a function of depth in tungsten.

Monte Carlo Simulation of X-Ray Fluence and Spectrum

The geometrical model of a transmission target for the Geant4 MCsimulation is shown in FIG. 3A. The MC model of x-ray source with atransmission target comprising a monoenergetic electron beam strikingthe tungsten-diamond composite target in a 1×1 mm² focal spot areaperpendicularly. The x-ray fluence was recorded in a detector positionedat a distance of 3 cm from the focal spot. MC simulations were performedfor different thicknesses of the tungsten target, while the thickness ofdiamond was kept at 2 mm.

For comparison, a reflection target was also modeled by MC simulation.FIG. 3B shows the model of the reflection target used in the Geant4 MCsimulation. The x-ray fluence is scored for a 15° anode angle 3 cm awayfrom the focal spot. Due to the anode angle, focal spot area is changedto 1×2 mm², for the reflection target model. For both reflection andtransmission target models, a 3 mm aluminum (Al) layer was used aslow-energy x-ray absorber to filter out the low energy photons that donot come out of the vacuum chamber of x-ray tube, to mimic filtration byvacuum envelope of an x-ray tube.

The x-ray fluence generated by the transmission target is expected toincrease with the thickness of tungsten until all electrons are stopped.However, the self-absorption of tungsten target also increases with thethickness of tungsten. Thus, the x-ray fluence of the transmissiontarget reaches a maximum at a particular thickness for a given electronbeam energy. MC simulations were performed for electron energies in therange 40-140 keV and x-ray fluence for different thicknesses of tungstentarget were calculated. On the other hand, the thickness of reflectiontarget has no effect on x-ray fluence, therefore thickness of thereflection target was not changed, and only a 5 mm thick W target wasmodelled.

Finite Element Transient Thermal Simulation of Target Temperature

To evaluate the focal spot power density limitation, finite elementthermal simulations were performed to study the focal spot temperatureand heat dissipation rate. Finite element models of the W-diamondtransmission target and W reflection target were built using aMultiphysics Finite Element Analysis (FEA) software. The FEA model ofW-diamond target comprises a 5 μm tungsten target and 2 mm thick diamondsubstrate on a copper base as shown in FIG. 2A. The focal spot wasmodeled as a multilayer heating element with the power as a function ofdepth generated by the MC simulation. A focal spot area of 1×1 mm² wasused in all the simulations of the transmission target. Only one-fourthof the actual volume was modeled because of the symmetry in the targetgeometry.

Temperature dependence of tungsten thermal conductivity and specificheat were included in the model. The temperature of the top and bottomsurfaces were kept constant at 373 K as the boundary condition, assumingthe tube is water cooled.

Transient thermal simulations were performed with different incidentelectron beam energies as the pulse-width varied from 50 μs to 3 ms. Theresulting transient temperature distributions of the x-ray focal spot atdifferent pulse widths and powers were calculated. The maximum allowedtube power while keeping the target temperature below 3000° C. for agiven pulse duration were determined.

A 5-6 μm W layer of the transmission target is suitable for x-raysystems having peak kilovoltages (kVps) in the ranges of 60-140, whichis commonly used for human imaging. Results indicated that the x-rayfluence of the transmission target can be 20-30% greater than that ofreflected x-rays with electron beams at the same energy deposited ontothe target. The W-diamond transmission target is able to achieve highpower operation under short pulse loadings. The W-diamond target enablesas much as a four-fold higher power or 8 times higher power density thanthe reflection target for the same temperature threshold.

Energy Deposition in the Target

The penetration of 80 keV, 100 keV and 120 keV electrons and theirenergy deposition as a function of depth in tungsten were modeled usingthe MC simulations described above. The results are shown in FIGS. 4Aand 4B. FIG. 4A shows the trajectories of 120 keV electrons in a 10 μmtungsten target. FIG. 4B shows Geant4 MC simulation results of energydeposition as a function of depth for 80 keV, 100 keV and 120 keVelectrons. The maximum depth of 120 keV electrons deposited in tungstenis less than 8 μm, which is about half of the continuous slow downapproximation (CSDA) range (described in tungsten-pyrolytic graphite(W-PG) laminate target). Therefore, a thinner tungsten layer can be usedfor the target. Furthermore, as shown in FIG. 4B, most of the electronenergy is deposited within the first few microns of the tungsten targetmaterial. The thickness of the tungsten layer can be further reduced.

Characteristics of X-Ray Beam Produced by W-Diamond Transmission Target

FIGS. 5A and 5B plot x-ray fluence and energy fluence produced byW-diamond transmission target as a function of tungsten thickness. FIG.5A shows transmission x-ray fluence from the W-diamond target as afunction of tungsten thickness recorded for different electron beamenergies. FIG. 5B shows energy fluence of transmission x-rays from theW-diamond target as a function of tungsten thickness recorded for 120keV electron beam energy. The Aluminum filtration removes low energyx-ray photons that would not come out of the vacuum envelope. x-rayfluence first increases to a maximum point and then decreases due toself-absorption by the tungsten target material. The energy fluencefollows the same trend.

FIG. 6 shows the thicknesses of tungsten layer that produces maximumx-ray fluence for different beam energies. The optimal thicknessincreases approximately linearly with electron energy.

An x-ray system may use different kVp settings in clinical imaging basedon the size of the subjects. The kVp setting used for imaging humansusually ranges from 60 kVp to 140 kVp. The efficiencies of 1-11 μmtungsten targets for transmission fluence were evaluated at differentelectron energies and the results are shown in FIG. 7. 100% efficiencyis defined as when the x-ray fluence is maximized for a given energy.The tungsten thickness of about 5-6 μm appears to be acceptable for thebeam energy in the range of 60-140 keV, where the x-ray fluence remainsabove 80% of its maximum value.

FIG. 8 shows the comparison of the photon spectra produced by bombarding120 keV electrons on a 5 μm W-diamond transmission and on a 5 mm thickreflection targets. Both x-ray beams are filtered by a 3 mm aluminumfilter. The results indicate that the Bremsstrahlung component of thetransmission targets is about 20% higher than the reflection target.However, the characteristic x-ray spikes of the transmission target aresignificantly lower than that of the reflection target. The lowercharacteristic x-ray component can be attributed to the energy thresholdof characteristic x-ray generation. Characteristic x-rays are generatedonly in the first few microns of tungsten, after which the electronslose their kinetic energy to produce characteristic x-rays. Thus,although the total numbers of characteristic x-ray photons are the samein transmission and reflection targets, the characteristic x-ray photonsare absorbed more in transmission target as they need to pass throughmore tungsten layers. Nevertheless, the total integral fluence of highenergy x-rays is still higher for the transmission target despite theadditional 2 mm diamond filter.

FIG. 9 shows x-ray fluence profiles as a function of the angle betweenthe x-ray and the target surface. The fluence is more uniform forreflection target than transmission target. However, the fluence of thereflection target at the central axis cannot be utilized. X-ray tubeswith a reflection target usually have a small anode angle. Thus only thefluence at large angle in FIG. 9 is used, which is called a heel effect.Transmission target, on the other hand, can utilize the photons incentral axis where the fluence is maximal. Although the flat region ofthe fluence for a transmission target is smaller compared with areflection target, this would not pose as a problem for x-ray imaging asonly a small angular window is used in x-ray systems using a reflectiontarget. The results indicate that, for a 120 kVp beam, the W-diamondtransmission target with a 5-6 μm W target can produce approximately 20%higher fluence than reflection target of the same tube power.

Transient Thermal Simulations

In transient thermal analysis, the focal spot was modeled as laminatedheating elements. The power of each heating element layer was assignedas a function of depths based on the MC results above. The finiteelement contains only ¼ of the anode to take advantage of the symmetryof the geometry. FIGS. 10A-10C show the temperature distribution of thetransmission target caused by a 3 ms pulse of 11 kW power (120 kV and91.7 mA). Enlarged views of the plots (pointed by the arrows) are alsoincluded FIGS. 10A-10C. FIG. 10A shows a schematic diagram of thetransmission target model used in FEA simulations. The FEA modelincludes ¼ of the transmission target, taking advantage of geometricsymmetry of the target. FIG. 10B shows temperature distribution near thefocal spot on the W-diamond transmission target surface caused by a 3ms, 11 kW beam calculated using a FEA simulation. FIG. 10C showstemperature distribution in the x-y plane of the target caused by a 3ms, 11 kW beam. The maximum temperature is observed at the center of thefocal spot as expected. The temperature decreases quickly outside thefocal spot and the gradient is very slow in the copper base.

FIGS. 11A and 11B show the temperature history of the focal spots forW-diamond transmission and W reflection targets, respectively. FIGS. 11Aand 11B shows variation of the maximum focal spot temperatures with timewith a 3 ms pulse (2% duty cycle) of (a) 11 kW electron beam power forthe transmission target (FIG. 11A) and (b) 1.9 kW electron beam powerfor the reflection target (FIG. 11B). The inserts in FIGS. 11A and 11Bshow enlarged plots of focal spot temperatures during the time intervalof 0-5 ms. The focal spot size of the transmission target and reflectiontarget are 1×1 mm² and 1×2 mm² respectively. For the transmissiontarget, the focal spot temperature rises very fast from 20° C. to 2500°C. during the first 0.5 ms, and to 3000° C. in 3 ms. When the electronbeam is turned off, the temperature drops rapidly to 280° C. in 5 ms. Atthe end of the 150 ms pulse cycle (assuming a pulse repetition rate of6.67 Hz), i.e., before the start of next pulse, the temperature drops to67° C. The results indicate that pulse mode operation of the tubeenables faster dissipation of heat with low duty cycle. Therefore, thebeam power allowable during the pulse duration may be kept significantlyhigh for short pulse widths. For the reflection target, the temperaturecurves rise continuously and do not result in a plateau compared to theW-diamond transmission target. Focal spot temperature also decreasesrapidly within 150 ms. But note the power of the electron beam is at 1.9kW, much lower than the power for the transmission target.

To keep the W-diamond target temperature spike below 3000° C., themaximum power allowed for different pulse widths were obtained and shownin FIG. 12. The maximum power is greatly affected by the pulse width.When the pulse width increases from 50 μs to 3 ms, the maximum power isreduced from 22 kW to 11 kW. Accordingly, in order to achieve high tubeoutput, x-ray sources with stationary anode should operate with shortpulse widths. The simulation also shows more than 22 kW peak power maybe achievable for pulse widths smaller than 50 μs.

For comparison, the same study was also performed for a 5 mm thickreflection W target with focal spot size of 1×2 mm² (the blue line inFIG. 12). The reflection target exhibits a similar trend, where themaximum power reduced as the pulse width increases. However, the maximumbeam power loading is only 14 kW at 50 μs, compared to 22 kW for aW-diamond target. The difference in maximum power is even larger forlonger pulses. When the pulse width is longer than 1 ms, the maximumpower of W-diamond transmission target can be four times higher than thereflection target. FIG. 12 also shows the maximum power calculated usingEq. (1) (the green line), which is valid for a thick target. Thesimulation and analytical calculation results for reflection target areconsistent with each other. Slight deviation may be due to the nonlinearthermal conductivity and specific heat used in the simulation model.

The focal spot power density of multi-pixel x-ray sources was simulatedby using a W-diamond transmission target. The transmission target designresults in advantage over a reflection target. A transmission target cansimplify the geometry of an x-ray tube as the x-ray beams are generatedon the opposite side of the cathodes. On the other hand, the x-ray beamsfrom the reflection targets come out between the cathode and the anode,where the space is usually very limited, thereby limiting the maximumfield size. There are significant amount of electrons scattered back tothe vacuum after bombarding the target. Back-scatter electrons carrylarge amount of energy and may add a long tail to the focal spot whenthey return to strike the target again. In transmission target, thex-ray generated by back-scattered electrons are largely absorbed by thetarget and blocked by the anode body.

The optimal thickness of the tungsten layer of W-diamond target islinearly proportional to the electron energy. A transmission targethaving approximately 5-6 μm thick tungsten may have the best x-rayproduction efficiency for the energy range of 60-140 kVp. Based on thesimulation calculation results, the W-diamond transmission target mayproduce about 20% more x-ray fluence for the same power compared with areflection target. The maximum power that keeps focal spot temperatureunder melting point is strongly dependent on the pulse duration. For apulse of a few ms, the power allowed by a W-diamond transmission targetcan be four-fold higher than a reflection target. Thus, it may allowsignificant improvement on the output of multi-pixel x-ray sources. Forexample, the power density limit of the 1 mm×1 mm focal spot when thesource operates with 50 μs pulses is as high as 22 kW/mm². Even thoughthe physical focal spot size of a reflection target is larger than theprojected focal spot due to the anode angle, a transmission target canstill achieve up to four times higher power despite its focal spot areais only half of that of the reflection target.

In another embodiment, a tungsten pyrolytic graphite (W-PG) laminatedtarget is provided. Due to its high melting point (3422° C.) and highatomic number (74), tungsten is a choice for x-ray source targetmaterial. But its relatively low thermal conductivity (173·W·m⁻¹·K⁻¹)significantly limits focal spot power density of x-ray sources. Themaximum depth of 120 keV electrons deposited in tungsten is ˜10 μm, thusa large amount of heat is deposited to a very thin layer of tungsten.Because of limited heat removal rates, the heat is built up quickly inthe target and may melt the tungsten if the tube power is too high. Toimprove the tube output, the target needs to have a higher thermalconductivity. Pyrolytic graphite (PG) is multiple layers of graphenesheets bonded together by covalent bonding. PG, especially annealedpyrolytic graphite (APG), has an exceptional high thermal conductivityof up to 1700·W·m⁻¹·K⁻¹ along its a-b plane, nearly 10 times higher thanthat of tungsten at room temperature. Similar to graphite, APG is alsovery refractory and can withstand up to 4000° C. in vacuum beforemelting, exceeding tungsten's melting point. Bremsstrahlung x-rayproduction efficiency is proportional to target atomic number. The low Znumber of carbon makes APG unsuitable as x-ray target material byitself. Nevertheless, by laminating W on APG, the high z number of W andhigh thermal conductivity of APG can be used to develop a novelcomposite anode that overcomes limitation of focal spot power density oftungsten targets.

A W-PG laminate target for MPTEX source as shown in FIG. 13A is inaccordance with one embodiment of the disclosure. FIG. 13A shows a W-APGlaminated target positioned on an APG anode base (FIG. 13A is not inscale). FIG. 13B shows thermal conductivity of APG compared to othertarget materials. a, b, and c are the crystallographic axes of thematerial. The thermal conductivity of APG in the a-b plane is more thanten times higher than W and, as a result, APG rapidly removes headinside the target. The APG anode body further conducts heat to the tubehousing. The thicknesses of APG and W layers are controlled such thatthe heat is evenly divided by multiple W layers. The APG layers embeddedbetween W targets quickly remove the heat due to their outstandingthermal conductivity.

An APG anode body with its thermal conductive a-b plane aligned with theheat conduction direction is also disclosed herein. Finite elementsimulation suggests that a 2-5 times increase of the focal spot powerdensity is possible with the new composite anode (see Example 1 below).The new anode fabrication technique disclosed herein can be used inmulti-pixel sources as well as single focal spot x-ray sources.

A novel target fabrication technique is disclosed herein to overcome thelimitations described above, and to enhance the performances of x-raysources. The described target fabrication technique includes laminationof W-PG targets and production of an APG anode base.

Lamination of W-PG Target:

Electron kinetic energy is deposited to a very thin layer (˜10 μm) of atungsten target. PG and W layers are laminated such that the total heatis divided to multiple targets at different depths. The thermalconductive APG layers embedded between the targets remove the heatrapidly due to its exceptional high thermal conductivity. Thisinnovative W-PG laminate target dramatically increases the focal spotpower density allowed.

APG Anode Base:

Graphite brazed with a tungsten layer is a common configuration forx-ray tube anodes. As shown in FIG. 13B, the thermal conductivity in thea-b plane of annealed pyrolytic graphite (APG) is 3-4 times higher thanregular graphite. APG has been employed in high-end electronics andaerospace that requires extreme cooling performance, but has never beenemployed in x-ray tube anodes. In multi-pixel x-ray sources, the heat isconducted primarily within the cross-section plane. When aligning APG'sa-b plane with the tube cross section, the extreme cooling performanceof APG can significantly improve the cooling rate of multi-pixel x-raysources. A rotating anode may also employ this technique with embeddedthermal vias to pass the heat to different APG layers.

Methods disclosed herein may be implemented on a computing device. FIG.18 is a block diagram of a computing device 1800. In the exemplaryembodiment, computing device 1800 includes a user interface 1802 thatreceives at least one input from a user. User interface 1802 may includea keyboard 1804 that enables the user to input pertinent information.User interface 1802 may also include, for example, a pointing device, amouse, a stylus, a touch sensitive panel (e.g., a touch pad, a touchscreen), a gyroscope, an accelerometer, a position detector, and/or anaudio input interface (e.g., including a microphone).

Moreover, in the exemplary embodiment, computing device 1800 includes apresentation interface 1806 that presents information, such as inputevents and/or validation results, to the user. Presentation interface1806 may also include a display adapter 1808 that is coupled to at leastone display device 1810. More specifically, in the exemplary embodiment,display device 1810 may be a visual display device, such as a cathoderay tube (CRT), a liquid crystal display (LCD), an organic LED (OLED)display, and/or an “electronic ink” display. Alternatively, presentationinterface 1806 may include an audio output device (e.g., an audioadapter and/or a speaker) and/or a printer.

Computing device 1800 also includes a processor 1811 and a memory device1812. Processor 1811 is coupled to user interface 1802, presentationinterface 1806, and to memory device 1812 via a system bus 1814. In theexemplary embodiment, processor 1811 communicates with the user, such asby prompting the user via presentation interface 1806 and/or byreceiving user inputs via user interface 1802. The term “processor”refers generally to any programmable system including systems andmicrocontrollers, reduced instruction set circuits (RISC), applicationspecific integrated circuits (ASIC), programmable logic circuits (PLC),and any other circuit or processor capable of executing the functionsdescribed herein. The above examples are exemplary only, and thus arenot intended to limit in any way the definition and/or meaning of theterm “processor.”

In the exemplary embodiment, memory device 1812 includes one or moredevices that enable information, such as executable instructions and/orother data, to be stored and retrieved. Moreover, memory device 1812includes one or more computer readable media, such as, withoutlimitation, dynamic random access memory (DRAM), static random accessmemory (SRAM), a solid state disk, and/or a hard disk. In the exemplaryembodiment, memory device 1812 stores, without limitation, applicationsource code, application object code, configuration data, additionalinput events, application states, assertion statements, validationresults, and/or any other type of data. Computing device 1800, in theexemplary embodiment, may also include a communication interface 1816that is coupled to processor 1811 via system bus 1814.

In the exemplary embodiment, processor 1811 may be programmed byencoding an operation using one or more executable instructions andproviding the executable instructions in memory device 1812. In theexemplary embodiment, processor 1811 is programmed to select a modelprovided by a user.

In operation, a computer executes computer-executable instructionsembodied in one or more computer-executable components stored on one ormore computer-readable media to implement aspects of the inventiondescribed and/or illustrated herein.

The order of execution or performance of the operations in embodimentsof the invention illustrated and described herein is not essential,unless otherwise specified. That is, the operations may be performed inany order, unless otherwise specified, and embodiments of the inventionmay include additional or fewer operations than those disclosed herein.For example, it is contemplated that executing or performing aparticular operation before, contemporaneously with, or after anotheroperation is within the scope of aspects of the invention.

When introducing elements of aspects of the invention or the embodimentsthereof, the articles “a,” “an,” “the,” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Although described in connection with an exemplary computing systemenvironment, embodiments of the invention are operational with numerousother general purpose or special purpose computing system environmentsor configurations. The computing system environment is not intended tosuggest any limitation as to the scope of use or functionality of anyaspect of the invention.

Embodiments of the invention may be described in the general context ofcomputer-executable instructions, such as program modules, executed byone or more computers or other devices. The computer-executableinstructions may be organized into one or more computer-executablecomponents or modules. Generally, program modules include, but are notlimited to, routines, programs, objects, components, and data structuresthat perform particular tasks or implement particular abstract datatypes. Aspects of the invention may be implemented with any number andorganization of such components or modules. For example, aspects of theinvention are not limited to the specific computer-executableinstructions or the specific components or modules illustrated in thefigures and described herein. Other embodiments of the invention mayinclude different computer-executable instructions or components havingmore or less functionality than illustrated and described herein.Aspects of the invention may also be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules may be located in both local and remotecomputer storage media including memory storage devices.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

EXAMPLES Example 1: Evaluation of the Multi-Pixel Thermionic EmissionX-Ray Source (MPTEX)

This Example describes the development of a multi-pixel thermionicemission x-ray source (MPTEX) prototype, and the evaluation of theperformance of the prototype.

Methods and Materials

Multi-Pixel Thermionic Emission X-Ray Source (MPTEX):

An MPTEX source prototype for TBCT applications is developed. The tubeis made of aluminum body with ConFlat flanges. Water cooling pipes areembedded in the aluminum tube body for fast heat removal. The anode ismade from a graphite bar brazed with a 5 mm thick tungsten target. FIGS.14A and 14B show the MPTEX prototype and its functional diagram. Thetube can contain up to 48 thermionic cathodes in a 4 mm spatial spacingand each cathode may produce the same numbers of focal spots. Both oxidecoated and dispenser cathodes that can produce 100 mA and 500 mA cathodecurrent respectively are evaluated. The tube is able to operate at 100kVp limited by the anode high voltage vacuum feedthrough. X-raymeasurement shows the target physical focal spot size is about 2 mm² andprojected focal spot is under 1 mm² with anode angle. In someembodiments, the tube current is limited under 100 mA and the dwellduration under 50 μs due to the limitation of focal spot power density.

Finite Element Analysis (FEA) Thermal Simulation:

To evaluate the performance of the anode, the heat distributions weresimulated using a thermal finite element simulation software.Additionally, the maximum power allowed was estimated by increasing thefocal spot power until the maximum temperature reaches about 3000° C.

Results

FIGS. 15A-15D show the results of thermal simulations. FIGS. 15A-15Cshow temperature distributions of tungsten target (15A), dual layer w-PGtarget with a low a-b thermal conductivity (15B), and dual layer w-PGwith a high a-b thermal conductivity (15C). 1 kW power was provided tothe targets and the right surface was kept at 500° C. FIG. 15D shows themaximum power allowed for the anodes while keeping focal spottemperature below 3000° C. Because thermal conductivity of PG variesgreatly depending on the synthesis and annealing processes, thus low(400 W·m⁻¹·K⁻¹) and high (1700 W·m⁻¹·K⁻¹) thermal conductivity valueswere used in the simulations. The W-PG laminate targets have two layersof tungsten that share the total power. The right surface is kept at500° C. as boundary condition. The thermal conductive a-b planes of PGin the target are perpendicular to anode surface, whereas in the bodythey are within the cross sectional plane. With 1 kW power deposited onthe same 2 mm² focal spots, the maximum temperature reaches about 2900°C. for conventional tungsten anode (FIG. 15A), 1725° C. for thecomposite anodes with a low PG thermal conductivity value (FIG. 15B),and 993° C. for the anode with a high PG thermal conductivity value(FIG. 15C) respectively.

As shown in FIG. 15D, the W-PG laminate targets can sustain a power ashigh as 5 kW, five times higher than a traditional tungsten target. Inthe simulation, the W-PG target is modeled as two 5 μm thick W layersthat equally share the power load. In reality, as many W layers asneeded can be synthesized using the chemical vapor deposition methoddescribed below in Example 2. The thickness of W layers can be as thinas the atomic level if needed. The result indicates that 2-5 timeslarger focal spot power density can be achieved with the new anodetechnique.

Example 2: Fabrication of a W-PG Laminate Anode on an APG Base

This Example describes the fabrication of APG anode modules with W-APGlaminate targets.

APG Base:

FIG. 16A shows the cross section of an MPTEX source and FIG. 16B showsthe anode module of the MPTEX source. The anode is about 25 cm long madefrom a graphite bar brazed with a 5 mm thick tungsten target. Because ofthe symmetric temperature distribution in tube length (z) direction, theheat flows primarily in the cross sectional plane. Thus, the a-b planeof APG with large thermal conductivity needs to be aligned parallel tothe tube cross section. The a-b plane of commercial APG platessynthesized with chemical vapor deposition (CVD) techniques isperpendicular to its thickness. Commercial APG plates can be purchasedand machined into the dog-bone shape as shown in FIG. 16B. The APGplates may be coated with W-PG laminate target and then stacked togetherwith the x-y planes of APG plates along the cross sectional plane (thedog-bone shaped plane). Each anode module plate can be 4 mm thickness,matching the cathode pixel spacing of MPTEX source. After W-PG laminatetarget is deposited on the lateral surface by the CVD technique (see thefollowing paragraphs), the anode module plates can be stacked togetherto form an elongated anode. As the heat only transfers in the a-b plane,the imperfect contact between APG plates would not affect the heatdissipation. High temperature thermal adhesive can be applied to the topand bottom surfaces to improve the contact with the ceramic insulators(see FIG. 16A). Similar thermal paste has been applied between thealuminum housing and ceramic insulator interfaces.

CVD Deposition of W-PG Laminate Target:

Chemical vapor deposition (CVD) is a widely used process for formingsolid materials, such as coatings, from reactants in the vapor phase.CVD deposition of tungsten on a graphite base is a technique used tofabricate x-ray tube anodes. PG or graphene is also produced in aprocess similar to tungsten CVD deposition, where hydrocarbon gas isheated until it breaks down into carbon. A CVD aperture for W and PGsynthesis is set up in the arrangement as shown in FIG. 17. W and PGdeposition can be switched by controlling the gas supplies.

The target deposited on anode with a CVD method can have a perfect ornear perfect contact with the anode base material. CVD of tungsten isusually carried out using tungsten hexafluoride, WF₆, which may bedeposited in two ways:

WF₆→W+3F₂,  (2)

WF₆→3H₂→W+6HF.  (3)

The byproduct HF is very corrosive, but is tolerable for PG, which isvery inert even in high temperature. The CVD reactions used to depositGP are based on the thermal decomposition of hydrocarbons. An exemplaryprecursor is methane (CH4), which is generally pyrolyzed at 1100° C. orabove, over a wide range of pressure from 100 Pa to 10⁵ Pa (1 atm). Thereaction in a simplified form is as follows:

CH₄→C+2H₂.  (4)

The thickness of tungsten or PG can be controlled by the depositiontime. The 4V-PG target may only cover the focal spot instead of theentire lateral surface of APG plates. An enclosure with only focal spotarea exposed is developed during CVD deposition of W. Once finished, theanode plate with laminate target may be examined with optical andscanning electron microscopes.

The graphene layers between W targets may have defect or mismatch afterCVD. Annealing at a high temperature (e.g. up to 3000° C.), producesmore planar and more uninform carbon structures that a low temperature,thus improving the thermal conductivity. The APG plate material isalready annealed at the factory, but the PG layers deposited on thetarget by CVD method may be annealed, which improves thermalconductivity from ˜400 to up to 1700 W·m⁻¹·K⁻¹. Although very thin, theAPG layers, embedded between tungsten targets, are important inimproving focal spot power density. After the laminate target isdeposited with CVD, the anode module is placed in an induction oven forannealing. The APG plate is held by a tungsten frame and placed in aquartz tube. The tube is sealed and vacuumed during annealing to preventoxidation. A disappearing-filament pyrometer is used to measuretemperature during annealing.

Example 3: Evaluation and Optimization of W-PG Laminate Target

This Example describes the development of a technique to evaluate W-PGtarget and optimize its performance.

Various parameters, such as number of W-PG layers, thickness of each PGand W layers, annealing temperature and time need to be determined. Thedesign is first guided by numerical simulation and then optimizedthrough experimental studies.

Optimization of W-PG Laminate Target Via Monte Carlo Simulation and FEAMethod:

The Continuous Slow Down Approximation (CSDA) range of a 120 keV beam isabout 15 μm. Thus, a total thickness of 10-15 μm for a W target issufficient. The heat should be evenly divided to different W layers toachieve the best result. Monte Carlo (MC) simulation is used todetermine the thicknesses of PG and W layers such that the powertransferred to each W layer is approximate the same. The energy lost tothe APG layers should be as low as possible. On the other hand, thinnerAPG conducts heat slower. Thus there is a balance between x-rayproduction efficiency and cooling performance. Thus MC simulation andFEA thermal simulation are used jointly to optimize the design. Theactual thermal conductivity of thin PG layer can vary greatly. Numericalsimulations are therefore used as a rough prediction, and the design isverified via experimental studies.

Experimental Studies:

After anode plates with W-PG laminated targets are fabricated, they areinstalled in the MPTEX tube to evaluate its performance. With a 100 kVanode voltage, the oxide cathodes can generate up to 100 mA current,enough to melt the tungsten target with a physical focal spot area ofabout 2 mm² (projected to 1 mm² by anode angle). To measure focal spottemperature directly, a viewport is installed on the MPTEX prototype asshown in FIG. 16A. Disappearing-filament pyrometer is not suitable formeasuring the temperature of a small spot. A pyrometer camera can bemounted on the viewport and used to measure temperature. The focal spottemperature can be estimated from the color of focal spots, whichchanges with temperature.

The focal spot temperature can be measured with a DC load first. A small10 mA current generates 1 kW focal spot power with a 100 kV anodevoltage. Cooling water flow is kept constant during the measurement. Theadvantage of using an MPTEX tube for evaluation is that multiple anodesamples can be tested in one setup. All anode modular pieces are 4 mmthick when stacked together, matching the cathode spacing. The focalspot temperature is compared with that of a standard 5 mm tungstentarget.

Once the optimized W-PG lamination configuration is obtained, damagingtests are performed to evaluate its extreme performance. The cathodecurrent is gradually increased while the water temperature is closelywatched. Residual gas analyzer (RGA) is used to monitor partial vaporpressure in the vacuum chamber. A sudden increase of W or carbon (C)vapor pressure indicates the breakdown of a focal spot. A molten focalspot can also be visualized through the viewport.

Assuming the new anode has a power density twice as much as a solidtungsten target and each focal spot can sustain 4 kW power (P), with amaximum water temperature rise of 80° C., the water flow rate F is:

$\begin{matrix}{F = {\frac{P}{C \cdot {\Delta T}} = {\frac{4000\mspace{14mu} {J \cdot S^{- 1}}}{4200\mspace{14mu} {J \cdot {kg}^{- 1}} \times 80\mspace{14mu} K} = {0.012\mspace{14mu} {{kg} \cdot {S^{- 1}.}}}}}} & (5)\end{matrix}$

For multi-pixel x-ray sources, the total power is nP, where n is thenumber of pixels. If n=50, a water flow rate of 0.6 kg/s or 9.5 gallonsper minute is needed to allow the MPTEX source to operate continuously.This is manageable with the MPTEX tube design described herein. As shownin FIGS. 14A and 16A, the MPTEX tube has four 8 mm diameter waterchannels run through the aluminum tube housing, which allows a waterflow up to 20 gallon per min.

What is claimed is:
 1. (canceled)
 2. A transmission target assembly,comprising: a substrate, wherein the substrate comprises: a firsttransmission surface; a second transmission surface opposite the firsttransmission surface, the second transmission surface including acollimator surface; a first side surface; and a second side surface, thefirst and second side surfaces being disposed between and connecting thefirst and second transmission surfaces; a target covering at least aportion of the first transmission surface of the substrate; and a base,wherein the base surrounds the first and second side surfaces and aportion of the second transmission surface such that the collimatorsurface of the second transmission surface and the target are exposed.3. The transmission target assembly of claim 2, wherein the substratecomprises diamond.
 4. The transmission target assembly of claim 2,wherein the target comprises tungsten.
 5. The transmission targetassembly of claim 2, wherein the transmission target assembly isconfigured to operate in a pulse mode.
 6. The transmission targetassembly of claim 2, wherein the transmission target assembly isoptimized by a Monte Carlo simulation.
 7. The transmission targetassembly of claim 6, wherein the Monte Carlo simulation comprises afinite element thermal simulation of a temperature distribution on thetransmission target assembly.
 8. The transmission target assembly ofclaim 2, wherein the target comprises a thickness in a range fromapproximately 5 μm to approximately 6 μm, and the transmission targetassembly is configured to receive electronic beams having an energy in arange from approximately 60 kVp to approximately 140 kVp.
 9. Thetransmission target assembly of claim 2, wherein the transmission targetassembly is configured to transmit x-rays through the target and throughthe substrate at the collimator surface of the substrate.
 10. Thetransmission target assembly of claim 9, wherein the base of thetransmission target assembly comprising a collimator edge, thecollimator edge forming a collimator and configured to guide the x-raystoward a fluence detector.
 11. A multi-pixel x-ray source, comprising:an anode including a target assembly, the target assembly comprising: asubstrate; and a target disposed on the substrate, the target comprisinga plurality of focal spots, wherein the anode is configured to emitx-rays from the plurality of focal spots; and a housing enclosing theanode, the housing further comprising a viewport disposed on thehousing, the viewport configured to estimate a temperature of theplurality of focal spots based on a color of the plurality of focalspots.
 12. The multi-pixel x-ray source of claim 11, wherein the targetassembly is a transmission target assembly.
 13. The multi-pixel x-raysource of claim 11, wherein the target assembly is a reflection targetassembly.
 14. The multi-pixel x-ray source of claim 13, wherein thesubstrate of the target assembly comprises annealed pyrolytic graphite(APG).
 15. The multi-pixel x-ray source of claim 13, wherein thesubstrate of the target assembly comprises pyrolytic graphite (PG), andan a-b plane of the PG of the substrate is aligned with a cross-sectionof the housing.
 16. A multi-pixel x-ray source having a plurality oftarget assemblies, each target assembly comprising: a target comprisinga plurality of tungsten layers laminated in pyrolytic graphite (PG); anda base comprising PG, the base including a lateral surface, wherein thetarget is disposed on the lateral surface of the base, wherein heat isconducted in a direction of starting from the target toward the base.17. The multi-pixel x-ray source of claim 16, wherein the PG comprisesannealed pyrolytic graphite (APG).
 18. The multi-pixel x-ray source ofclaim 16, the PG including an a-b plane, wherein the a-b plane of the PGis aligned with the heat conduction direction of each target assembly.19. The multi-pixel x-ray source of claim 18, wherein each targetassembly comprises an APG plate having its a-b plane perpendicular to adirection of its thickness, and the plurality of target assemblies arestacked together along the direction of the thickness of the APG plate.20. The multi-pixel x-ray source of claim 16, wherein each tungstenlayer of the target is deposited in the PG by chemical vapor deposition(CVD).